In general, amplifiers that amplify transmission signals are provided in wireless communication apparatuses, such as base station apparatuses. Normally, if an input level to an amplifier is relatively small, the linearity of an input and an output of signals is maintained, whereas, if an input level to an amplifier increases and an output level thus approaches the saturation level, an input and an output of signals become nonlinear due to inter modulation distortion. Consequently, when an amplifier is operated at around the saturation level in which the power efficiency is high, the amplifier needs to be operated in a nonlinear region in which an input and an output of signals become nonlinear.
When an amplifier is operated in a nonlinear region, it is preferable that a distortion compensation circuit that compensates the inter modulation distortion generated in the amplifier be provided in a wireless communication apparatus. Specifically, by compensating, using a distortion compensation circuit, inter modulation distortion that is generated at the time of amplification, it is possible to maximize the performance of the amplifier. In recent years, predistortion circuits are widely used as distortion compensation circuits.
A predistortion circuit compensates inter modulation distortion by previously distorting an input signal that is input to an amplifier. Specifically, distortion that has the inverse characteristic of inter modulation distortion generated in the amplifier is previously given to an input signal, distortion of the input signal is canceled out by inter modulation distortion that is actually generated in the amplifier and thus the input/output linearity is improved. When distortion given to an input signal is calculated, in some cases, for example, a memory polynomial calculation is performed. Specifically, by performing the memory polynomial calculation, a feedback signal from an amplifier is compared with an input signal to the amplifier and then distortion given to the input signal is adjusted.
Patent document 1: U.S. Pat. No. 6,437,644
However, in order to perform the calculation that uses the memory polynomial, there is a problem in that a circuit becomes complicated and the size of the device and the cost are increased. Specifically, when, for example, a feedback signal from an amplifier is represented by a memory polynomial, a bit resolution in accordance with the level of a feedback signal is needed. At this point, because the dynamic range of the feedback signal is relatively large, the number of bits that represents the level of the feedback signal tends to increase. Consequently, because a circuit that is used to perform predistortion becomes complicated or the processing amount of the circuit increases, the size of the device and the cost increase.